A penis is the male sexual and reproductive organ, consisting of a head, called the glans, and the shaft or body. A fold of skin called the foreskin covers the glans. The foreskin is like a hood and can be rolled back to expose the head of the penis. The body or shaft of the penis is made up of three cylinders of soft, spongy tissue, which contain many small blood vessels. The entire penis contains many nerve endings that make it sensitive to touch, pressure, and temperature. The glans, however, has a higher concentration of nerve endings than the shaft of the penis and is thus particularly sensitive to physical stimulation. Two other areas that are highly sensitive are the rim that separates the glans from the shaft of the penis, called the coronal ridge, and the small triangular region on the underside of the penis where a thin strip of skin called the frenulum attaches to the glans. During sexual excitement, the blood vessels of the spongy tissue fill with blood and swell up. The rapid, forceful dilation of these blood vessels throughout the entire penis causes it to become firm and grow in size. This transition from a soft (flaccid) penis to a harder, stiffer penis is called getting an erection. The penis usually returns to a softer (non-flaccid) state shortly after ejaculation or after the penis is no longer being stimulated. The erect penis is inserted into a woman's vagina during the sexual act called intercourse or coitus. When a man reaches the pinnacle of sexual excitement, his orgasm is often accompanied by spurts of semen from the opening at the tip of the glans. This is called ejaculation. The semen that is released from the penis during ejaculation contains sperm, the reproductive cell capable of fertilizing an ovum or egg, the female reproductive cell. When one of the millions of sperm encounters an egg inside a woman's body, their union can produce a pregnancy. The appearance of the penis varies considerably from one male to another. There can be differences in color, shape, size, and the status of the foreskin (circumcised or uncircumcised). A man's weight, build and height bear no relation to the size of his penis in either the soft or erect state, nor is penis size related to his foot, hand or nose size. Art and the media, particularly men's magazine's and erotic books and movies, often portray male genitals in "larger than life" dimensions, giving men an unrealistic standard of comparison that can contribute to their concerns about their penis size. Concerns about penis size are common among men of all ages. When a penis is soft it usually hangs loosely away from the body and averages about 3.5 to 4.5 inches in length and one inch in diameter, though some are smaller and some are larger. The same penis can vary in size even when soft. For instance, cold air, cold water, fear, anger or anxiety causes the penis, scrotum and testicles to be pulled closer to the body, thereby shortening it. By the same token, a soft penis can actually lengthen in warm conditions and when a man is completely relaxed. Although the size of the non-erect penis differs widely from one male to another, this variation is less apparent in the erect state. Masters, Johnson and Kolodny (1986) in their book Sex and Human Loving state that erection can be thought of as "the great equalizer" because men with a smaller non-erect penis usually have a larger percentage volume increase during erection than men who have a larger flaccid penis. The great interest in penis size is related to several different things. First, it shows a concern for being "normal" - the same as everyone else, or certainly no worse. Second, it is related to a wish to be sexually adequate. Many people in our society believe that "bigger is better" and the myth that a big penis will provide more sexual satisfaction to a woman is widespread. Actually, penile diameter has little physiological effect for the woman because the vagina can gradually adjust to fit a penis of any circumference (remember, it is designed, under the right conditions, to allow a baby to pass through the same opening). The length of the penis, which determines the depth of vaginal penetration, is relatively unimportant, because it is the first one-third of the vagina that has the most nerve endings and is most responsive to physical stimulation. Penis size may, however, have positive or negative psychological significance to a woman. Some women prefer a large penis; others are put off by what they think is "too large" or "too small". Third, men may feel that a larger penis gives them an element of status and makes them more sexually attractive.

Sometimes, shortly after birth, the foreskin is removed from the penis in a procedure known as circumcision. The decision to perform a circumcision is often made for religious reasons, and in some rare cases it is necessary in order to release a foreskin that is too tight around the penis. Many physicians and parents also feel removal of the foreskin promotes better hygiene, although with little evidence to support this claim, some now choose to forego circumcision of infant boys.